The origin of biotite in Archaean meta-sediments near Yellowknife,N.W.T., Canada: A reply to E. H. Brown

Conclusion Dr. Brown's perceptive criticism has emphasised that problems of detail remain and pointed out some deficiencies of presentation in my paper. I hope these have now been rectified. I believe, however, that the above examination of the reaction in the light of his criteria has served only to substantiate its general validity.     

The cordierite isograd in archaean meta-sediments near yellowknife,N.W.T., Canada-variations on an experimentally established reaction

In the Slave Craton, meta-sediments commonly exhibit broad, Abukuma-type metamorphic aureoles round late, potassic plutons. The rocks are divided into low and medium metamorphic grades by sharply-defined cordierite isograds. Comparison of mineral assemblages above and below one such isograd indicates that chlorite and muscovite were reactants. Biotite is present below the isograd, and textural relations suggest that it participated in the reaction. Micro-probe analyses of biotites in cordierite-free and cordieritebearing rocks, considered in the light of established pro-grade compositional changes, indicate that the biotite changed in composition during the reaction, supplying Si in exchange for Al^IV, Fe and Mg in exchange for Al^VI, and K in exchange for Na. The reaction can be balanced in a fashion consistent with all the observable evidence. The balancing operation indicates that abundant quartz and traces of albite and ilmenite also reacted, and that biotite became more abundant during the reaction. Thus the complex reaction can be approximated by the formula Chl+Musc+Biot(1)+Qtz [+Albite+Ilm] →Cord+Biot(2)+Water It is of a type extensively studied in experimental haplo-pelitic systems but was modified in the natural system mainly because additional phases participated. In particular, pre-existing biotite was compositionally modified and increased in amount during the reaction.     

Temporal Evolution and Petrogenesis of an Archaean Crustal Segment at Yellowknife, N.W.T., Canada

Geochronology of several Archaean rock units in the Slave Provinceestablishes a sequence of events which may be treated as a singletectonic cycle, approximately 75 m.y. in duration, leading tothe formation of a new crustal segment. The oldest rocks (2650m.y.) are basic metavolcanics with largely calc-alkaline characteristics.Metamorphic adjustment to greenschist facies conditions is largelyisochemical but departures from regular serial variation inalkalis are noted. Comparison with experimental studies suggeststhat the basalt-dacite volcanic association is probably theearly extrusive expression of the subcrustal processes whichled to the intrusion of calc-alkaline batholiths into the deformingvolcanic-sedimentary pile. Zircon U-Pb and whole rock Rb-Sr isochron ages of 2620–2640m.y. are recorded for the syn-kinematic quartz diorite batholithsand 2590–2610 m.y. for the major late-kinematic bodies.Pegmatitic adamellites (2575±25 m.y.) are the youngestplutonic units. Regional metamorphism of the andalusite-sillimanitefacies series reflects the relative unimportance of load pressurein the thin-crustal Kenoran deformation. Comparison of the resultsobtained by the three radiometric methods confirms that an intervalof over 100 m.y. may elapse between the time of emplacementof a given rock unit and the subsequent closure of the biotiteK-Ar system. The sequence of K-Ar mineral dates is in accordwith the relative argon retentivities of the various minerals.     

Oxide and sulphide mineralogy of the Peuyuk kimberlite,Somerset Island,N.W.T., Canada

The Peuyuk kimberlite is divisable into three petrographically distinct phases (A, B, and C) on the basis of oxide mineralogy and the presence or absence of an immiscible carbonate liquid.Phase A contains spinels (titan-magnesian-aluminous chromite) and perovskite with no reaction rims or complex mantles, and no evidence of an immiscible carbonate liquid.Phase B contains complexly zoned and mantled oxides with no evidence of an immiscible carbonate liquid. The spinels are zoned from titan-magnesian-aluminous chromite to members of the magnesian ulvospinel-ulvospinel-magnetite series. The zoned spinels are mantled by a serpentine-carbonate mixture followed by a mixture of Ti free magnetite and minor rutile. Perovskites are mantled by nickeliferous pyrite (0–11 % Ni) and rutile.Phase C is characterized by the presence of an immiscible carbonate liquid. Spinels are either discrete crystals of magnesian ulvospinel-ulvospinel-magnetite or as discrete rims of this composition upon cores of titan-magnesian-aluminous chromite. Rutile rims are poorly developed upon perovskites. Sulphides associated with the silicate groundmass are Cu-pentlandite, heazle-woodite and chalcopyrite. Nickeliferous pyrite is associated with the carbonate fraction.Common to all phases of the intrusion is a red titanium poor aluminous-magnesian chromite which is interpreted to have formed prior to the fluidized intrusion of the kimberlite. All other spinels being considered to have crystallized after fluidization.The spinel composition trends may reflect decreasing oxygen fugacities (10^–19 to 10^–22 bars) in response to falling temperatures (800–600° C) along the quartz-fayalite-magnetite buffer.In the immediate post-fluidization history of the diatreme all phases crystallized titan-magnesian-aluminous chromite but the trends of Ti and Fe enrichment observed in phases B and C was prevented in phase A possibly by rapid cooling. Slower cooling coupled with reaction of earlier formed phases with residual H₂O and CO₂ rich fluids resulted in the phase B assemblage. Extensive reaction of this type was prevented in phase C by the separation of an immiscible carbonate liquid.     

Composition and reactivity of some river sediments from the Mackenzie Valley,N.W.T., Canada

Sediments from the Mackenzie Valley — two suspended and one river bottom — were analyzed for major and minor elements, mineralogy, particle size, and ability to release trace metals when subjected to mild leaching. Copper and zinc in suspended sediments and cadmium in bottom sediment occurred largely (70 to 84 percent for Cu, 72 to 98 percent for Zn, 75 to 81 percent for Cd) in a form that could not easily be separated from the sediments by 0.1M HC1 and EDTA solutions. Cadmium was present in suspended sediments in low concentrations, and was present totally in sorbed leachable form. Iron, manganese, cobalt and chromium were separable from the particulate phase by mild leaching. It was found that approximately 70 hours were required for an apparent steady state concentration to be attained in such leachings. Reaction of these sediments with Beaufort Sea water (salinity 22.3 parts per thousand) indicated that both iron and manganese were released from sediments to sea water to a very small degree. Copper, lead, and zinc, however, were absorbed by sediments from sea water.     

Mineralogy,paragenesis and origin of the Ag-Ni,Co arsenide mineralisation,Camsell River,N. W. T. Canada

Polymetallic hydrothermal vein mineralisation of the Ag-Ni, Co Arsenide type, and containing Bi and U, occurs in three mines and five uneconomic veins in the Camsell River area. Each occurrence is described in detail. The veins lie in sulphide-impregnated volcanic rocks of the Aphebian Great Bear Batholith. Large, NE-trending strike-slip faults were initiated during the terminal phases of orogeny (1750 m.y.), and were active intermittently for at least 300 m.y. The veins are located in splays and tension-fractures form the primary faults, and mineralised lenses are found in dilatant zones in these structures. The mineralisation shows a generalised paragenesis of U → Ag+arsenides → Bi, Sulphides, Sulphosalts, but there are variations. A model is presented to explain the temporal and spatial controls of mineralisation and to explain the perturbations in the paragenesis. It is concluded that the mineralisation is of magmatic origin and lateral secretion models are rejected. Discussions of the ultimate origin of the ore elements are not presented here.     

Chemical mineralogy of some cordierite-bearing rocks near Yellowknife,Northwest Territories,Canada

Meta-graywacke and meta-argillite of Archean age near Yellowknife contain biotite, cordierite, gedrite and sillimanite isograds towards the Sparrow Lake granite pluton. The chemistry of biotite, cordierite, gedrite and garnet in rocks that up-grade from the cordierite isograd indicate a small range of chemical composition, particularly with reference to Mg, Fe and Mn. The analyses show further that among the coexisting ferromagnesian minerals Fe/Fe+ Mg ratio decreases in the sequence: garnet, gedrite, biotite, cordierite while Mn/Fe+Mg+Mn ratio decreases in the sequence garnet, gedrite, cordierite, biotite. The same order is also observed in the distribution diagrams. The regular distribution of Mg, Fe and Mn among the coexisting phases demonstrate that chemical equilibrium was attained and preserved in these Archean rocks. Mg-Fe distribution between cordierite and biotite appears to be dependent on the temperature of crystallization or metamorphic grade.     

Late Quaternary glacial and climatic history of northern Cumberland Peninsula,Baffin Island,N.W.T., Canada

Radiocarbon dates on molluses in marine facies associated with glacial deposits in northern Cumberland Peninsula indicate both main fiord (Laurentide) ice and local glaciers remained at their late Wisconsin maxima until ca. 8000 BP. Essentially continuous deglaciation followed; local corrie glaciers melted out by 7100 BP and by 5500 BP fiord glaciers had receded behind the present margin of the Penny Ice Cap. The Hypsithermal warm interval probably lasted from ca. 8000 to 5000 BP. Lichenometry and radiocarbon dates on peat and buried organic horizons delimit a detailed Neoglacial chronology. Of 46 outlet and corrie glaciers investigated, the oldest Neoglacial moraines are dated lichenometrically at 3200 ± 600 BP. Subsequent advances terminated immediately prior to ca. 1650, 780, 350, and 65 yr BP, the most recent of which marked the most extensive ice coverage during the Neoglacial. The highest occurrence of lateral moraines from late Wisconsin advances of local and Laurentide ice suggest that at the late Wisconsin glacial maximum, depression of snowline varied from 450 m below present at the coast to 350 m below present level in the vicinity of the Penny Ice Cap. Moraines, surrounded by glacial ice and lying above the present steady-state ELA, suggest that during the Hypsithermal snowline was up to ca. 200 m above its present elevation. A radiometrically controlled reconstruction of relative summer paleotemperatures for the postglacial derived independently of lichenometry agrees well with the lichenometric age dating of moraines. The data suggest that between ca. 1650 and 900 BP climatic conditions were unfavorable for glacier growth, whereas the period ca. 800-65 yr BP was one of general glacial activity. During the last decade permanent snow cover has been increasing in the area. Previously reported data on climatic trends in the Canadian Arctic based on palynological analyses are similar to the chronology reported here.     

Growth of cordierite in the Archean metasediments near Yellowknife,district of Mackenzie,Canada

The widespread occurrence of cordierite in the Archean metasediments near Yellowknife was attributed by earlier workers to the contact metamorphism associated with the granite. However, detailed field and textural studies on the cordierite-bearing rocks near Sparrow Lake indicate that the growth of cordierite is not restricted to the aureole around the Sparrow Lake granite. Fabric relations demonstrate that cordierite grew under regional metamorphic conditions existing before and after the intrusion of the granite. Emplacement of the Sparrow Lake pluton is considered to represent the culmination stage of regional tectonism that manifested itself as deformation, metamorphism and granite intrusion in the Sparrow Lake area.

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Zusammenfassung Die Verbreitung von Cordierit in den archaischen Metasedimenten bei Yellowknife führten frühere Autoren auf die mit dem Granit verbundene Kontaktmetamorphose zurück. Genaue Gelände- und Strukturuntersuchungen an den cordieritführenden Gesteinen aus dem Gebiet von Sparrow Lake zeigen jedoch, daß das Wachsen von Cordierit nicht auf die Aureole um den Sparrow Lake-Granit beschränkt ist. Gefüge-Beziehungen beweisen, daß Cordierit unter regionalen metamorphen Bedingungen gebildet wurde, die vor und nach der Intrusion des Granits herrschten. Die Bildung des Sparrow Lake-Plutons wird als Höhepunkt regionaler tektonischer Vorgänge angesehen, die im Sparrow Lake-Gebiet als Deformation, Metamorphose und Granitintrusion in Erscheinung traten.

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Résumé L'extension de la cordiérite dans les métasédiments archéens de la région de Yellowknife fut longtemps attribuée au métamorphisme de contact associé au granite. Cependant des études précises sur le terrain et l'examen de la texture des roches à cordiérite indiquent que la croissance de la cordiérite n'est pas restreinte à l'auréole du granite du lac Sparrow. Les relations vectorielles démontrent que la croissance de la cordiérite s'est effectuée dans des conditions métamorphiques régionales existant avant et après l'intrusion du granite. La mise en place du pluton du lac Sparrow réprésenterait donc l'étape culminante de la tectonique régionale qui dans la région du lac Sparrow s'est manifestée par la déformation, le métamorphisme et l'intrusion granitique.

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, . Yellowknife , Sparrow-Lake. , , . Sparrow-Lake , , , .

     

Late quaternary glaciation of the Eastern Queen Elizabeth Islands,N.W.T., Canada: Alternative models

It has been suggested that during the last glaciation the Innuitian Ice Sheet existed over the eastern Queen Elizabeth Islands. This is based on the pattern of postglacial emergence over this area and the timing of driftwood penetration into the interisland channels. Alternative interpretations of both sets of data raise questions about the presence of the Innuitian Ice Sheet at this time. Field observations on northeastern Ellesmere Island, plus additional data pertaining to the presence of multiple tills and “old” radiometric dates on lacustrine deposits, shelly tills, and raised marine features suggest that the maximum glaciation over this region, equivalent to the Innuitian Ice Sheet, predates the last glaciation, Palaeoclimatic conditions are also discussed in relation to these data. It is suggested that during the last glaciation of the Queen Elizabeth Islands there was a convergent but not coalescent advance of the existing upland ice-fields. This noncontiguous ice cover over the Queen Elizabeth Islands is termed the Franklin Ice Complex. It is suggested that the term Innuitian Ice Sheet be reserved for contiguous older glaciations over this same area.     

The behaviour of platinum group elements in the surficial environment at Ferguson Lake, N.W.T., Rottenstone Lake, Sask. and Sudbury, Ont., Canada

A multidisciplinary geochemical study of the distribution, dispersion, and glacial dispersal, of the pge and associated elements has been undertaken within soil, till, humus, vegetation and water at Ferguson Lake, Northwest Territories, Rottenstone Lake, Saskatchewan and Sudbury, Ontario.As the pge generally are present at low levels in surficial materials, development work on analytical techniques was an essential part of this study.At Ferguson Lake, the spatial distribution patterns of Au, Pt and Pd in till clearly indicate the exposed gossan zones, as expected, but also indicate a possible extension of the zone beneath a peat bog- and till-covered area. Down-ice dispersal of Au, Pt and Pd is limited to one to two hundred metres, in the <63 μm component of the till samples collected from frost boils. In vegetation the pge enrichment extends for several hundred metres down-ice and is best defined by Pd in birch twigs. Detectable, although extremely low, levels of Pt (2.8 ppt) and Pd (2.0 ppt) are present in waters in the vicinity of the gossanous zones at Ferguson Lake.At Rottenstone Lake, moderate to high concentrations of pge, Au, and base metals were found in ashed twigs of black spruce and the hmc of the tills for a distance of less than two hundred metres down-ice of the mineralization. Low Pd and Au concentrations were present in ashed spruce twigs about one kilometre down-ice of the mineralization, where only the hmc of the tills yielded anomalous concentrations of Pt and Au. There appears to be only limited dispersal of the pge and Au. These data indicate that only the hmc and the spruce twigs are of value in detecting Pt and Au in this area. Palladium presents a different picture, being detectable in only some of the soils, absent in the tills and hmc, yet appreciably enriched in the twig ash. The inference is that Pd is moving in solution and is being somewhat adsorbed in the soil but is much more significantly being taken up by the plant roots.At the Sudbury areas the pge, hosted in the Ni-Cu mineralization, are best reflected by elevated levels in the ashed humus of almost all elements examined. There is only minimal response in pge and Au to the mineralization from any of the fractions of the soil; whereas the <2 μm fraction of the B-horizon soil reflects the mineralization by elevated levels of As, Sb, Se, Cr, Co, Ni, Cu, Pb and Zn. Only hmc from the tills show elevated pge, Au and variable enhancement in As, Sb, Se and the base metals. The < 2 μm portion of the tills tends to be highest in As, Se, Cr and the base metals.This ongoing study shows that surficial materials and vegetation are effective in identifying areas of concealed pge mineralization. Various pathfinder elements, primarily Cu and Ni, but perhaps also As, Se, Sb and the other base metals, in the < 2 μm B-horizon soils and tills, may be informative in a preliminary evaluation of the pge potential of an area, prior to undertaking the more expensive precious-metal analyses. Humus and vegetation both appear extremely effective, and most cost efficient, and heavy-mineral concentrates (hmc) appear effective, for identifying areas with pge potential, whereas hmc from tills appear most effective for zeroing in on the site of the pge mineralization.     

Primitive Magma From the Jericho Pipe, N.W.T., Canada: Constraints on Primary Kimberlite Melt Chemistry

We report the first estimates of primary kimberlite melt compositionfrom the Slave craton, based on samples of aphanitic kimberlitefrom the Jericho kimberlite pipe, N.W.T., Canada. Three samplesderive from the margins of dykes where kimberlite chilled againstwall rock (JD51, JD69 and JD82) and are shown to be texturallyconsistent with crystallization from a melt. Samples JD69 andJD82 have geochemical characteristics of primitive melts: theyhave high MgO (20–25 wt %), high mg-numbers (86–88),and high Cr (1300–1900 ppm) and Ni (800–1400 ppm)contents. They also have high contents of CO₂ (10–17 wt%). Relative to bulk macrocrystal kimberlite, they have lowermg-numbers and lower MgO but are enriched in incompatible elements(e.g. Zr, Nb and Y), because the bulk kimberlite compositionsare strongly controlled by accumulation of mantle olivine andother macrocrysts. The compositions of aphanitic kimberlitefrom Jericho are similar to melts produced experimentally bypartial melting of a carbonate-bearing garnet lherzolite. Onthe basis of these experimental data, we show that the primarymagmas from the Jericho kimberlite could represent 0·7–0·9%melting of a carbonated lherzolitic mantle source at pressuresand temperatures found in the uppermost asthenosphere to theSlave craton. The measured CO₂ contents for samples JD69 andJD82 are only slightly lower than the CO₂ contents of the correspondingexperimental melts; this suggests that the earliest hypabyssalphase of the Jericho kimberlite retained most of its originalvolatile content. As such these samples provide a minimum CO₂content for the primary kimberlite magmas from the Slave craton. KEY WORDS: kimberlite; melt; primitive; primary magma; Slave craton     

Petrology and Li-Be-B geochemistry of muscovite-biotite granite and associated pegmatite near Yellowknife,Canada

Prosperous granite (Rb-Sr 2520±25 Ma) occurs as several plutons (1–380 km² outcrop area) in a thick succession of metamorphosed greywacke-mudstone of the Yellowknife Supergroup. The average mineral content of the Sparrow pluton (in vol.%) is quartz (32), plagioclase (31), K-feldspar (24), muscovite (9), biotite (3), and apatite (<1). Average trace-element concentrations (in ppm) are Li (140), Be (4), B (28), Zn (47), Rb (250), Sr (76), Zr (75) and Ba (360). The central portion of the pluton is slightly richer in K, Sr, and Ba than the margin. Li is concentrated in mica (Li in biotite/Li in muscovite=4.7), and Be and B in muscovite and plagioclase. Countless pegmatite dikes occur in the Sparrow pluton and in schist-hornfels to the east; the outer limit is marked by the cordierite isograd, 9 km from the granite contact. Dikes vary greatly in size (1 km to a few cm in length), in mineral content (quartz, albite, K-feldspar, muscovite, tourmaline, beryl, spodumene), in major element composition (especially the NaK ratio), and in trace-element content (Li 18–5000 ppm, Be 5–260 ppm, B 20–150 ppm). Compared with Prosperous granite, the pegmatite bodies are richer in P and Rb, and poorer in Ti, Fe, Mg, Zr, and Ba. Dikes rich in tourmaline, beryl, and spodumene occur in overlapping zones situated progressively farther from the centre of the Sparrow pluton. The composition of tourmaline is related to host rock; the highest concentrations of Fe and Zn occur in crystals from pegmetite and the highest concentrations of Mg and V occur in crystals from tourmalinized schist, while those from granite and quartz veins occupy on intermediate position. Complex compositional zoning is present in some tourmaline crystals in pegmatite. Estimates of temperature (500°–600° C) and pressure (2–4 kb) of granite emplacement, based on the distribution of andalusite and sillimanite in the contact rocks, suggest that the final stage of granite emplacement occurred at sub-solidus conditions. A vaportransport model is proposed to explain the widespread distribution of the pegmatite dikes and their extreme compositional variability. Some of the pegmatite constituents, including Li, Be, and B, were possibly derived from Yellowknife graywacke and mudstone.     

Controls of Biotite Zone Mineral Chemistry in Archaean Meta-Sediments near Yellowknife, Northwest Territories, Canada

Archaean meta-sediments near Yellowknife, Canada, exhibit alow-pressure facies series and broad metamorphic zonation rounda central pluton. Meta-pelites and meta-greywackes from thebiotite zone have been studied using analyses of 59 mineralsand 14 rocks. Mineral compositions were controlled by both host-rockcomposition and metamorphic grade. Increased grade commonly imposed on the minerals a progressivecompositional maturation. This involved progressive compositionalchange (especially in meta-greywacke biotites) and/or narrowingof compositional range (particularly in muscovites). Specificeffects of increased grade are as follows. Biotites in meta-greywackesexhibit increased Mg/Fe and Na/K but decreased (Na+K) content.Biotites in meta-pelites change little except for increasedAl^IV/Si. The muscovites display decreasing maximum Si contents,increased Al^VI at the expense of Fe and Mg, and increased Na/K.The chlorites show only a slight general increase in Mg/Fe. Bulk compositional control is manifested in various ways. Insome instances minerals from subtly different rock types areperceptibly distinct; in others the pro-grade maturation trendsof minerals in these rock types are different. Thus Si is moreabundant in meta-greywacke biotites than in meta-pelite biotitesand (Na+Ba+K) content of muscovites is greater in meta-greywackes. The observed features are used to deduce mechanisms of controlby grade and bulk composition and to discuss general circumstancesunder which they operate.     

A garnet population in Yellowknife schist, Canada

Abstract Data are presented on a garnet population in a specimen of garnet-biotite-plagioclase-quartz schist from the cordierite zone of an Archaean thermal dome in the Southern Slave Province of the Canadian Shield. Garnet crystals are bounded by planar dodecahedral faces and by trapezohedral faces which on the 10-μm scale are corrugated. Crystal distribution, as revealed by dissection of a small cubic volume of rock, is random. The size distribution is normal, with a mean diameter of 0.81 mm and a standard deviation of 0.32 mm. In the largest crystal of the population (mean radius 0.83 mm), [Mn] = 100 Mn/(Fe + Mg + Mn + Ca) decreases from 14.5 at the centre to 7.5 and then increases in the outer margin to 8.5; [Fe] increases continuously from 67 at the centre to 77 at the surface; [Mg] increases from 12.5 to 13.5 and then falls sharply to 11; [Ca] remains unchanged at 4.0 and then drops to 3.3. Progressively smaller crystals have progressively lower [Mn] and higher [Fe] concentrations at their centres, while all crystals have the same margin composition. Growth vectors extending from given concentration contours to crystal surfaces are of equal length regardless of the size of the crystal in which the vector is located. A garnet-forming model is presented in which reaction was initiated by a rise in temperature. Nucleation sites were randomly selected. The nucleation rate increased with time and then declined. Crystal faces advanced at a constant linear rate, which implies an increase in volume proportional to surface area. Initially, the composition of garnet deposited on crystal surfaces was determined by van Laar equations of equilibrium, which demanded the withdrawal of Mn and Fe from within chlorite crystals. This transfer reaction was then accompanied by an ion exchange reaction which moved Mn and Fe to garnet surfaces from biotite, in exchange for Mg. The exchange reaction provides an explanation for the high overall concentration of Mn and Fe in garnet and for the observed Mn and Mg reversals in the margins of crystals. The increase of garnet volume in the garnet population is found to be parabolic, i.e. Vαα⁵.     

Variations in the size of coarse clastic particles over the surface of a small sandur, Ellesmere Island, N.W.T., Canada

For a small sandur in the Canadian Arctic, trend surface analysis of fifty-three samples shows that less than 40% of clast size variation is explained by downsandur fining. Further variations are explained by considering different levels of the sandur surface separately. In the main flow area, which carries water even during periods of low flow, channel deposits are invariably coarser than bar deposits although the difference lessens and overall sorting improves with distance downsandur. The flood surface area, which carries water only during high discharges, is less affected by downsandur fining than the rest of the sandur, and the bar-channel relationship is weaker. The high flood surface which occupies most of the proximal zone of the sandur is probably an abandoned main flow surface, as indicated by clast size characteristics similar to those on the present main flow surface.     

Metasomatic crystallization of muscovite in granite and tourmaline in schist related to pegmatite emplacement near Yellowknife,Canada

Numerous pegmatite dikes occur in the Sparrow pluton (muscovite-biotite granite) and in the adjacent cordierite-zone schist-hornfels of the Yellowknife Supergroup. Where pegmatite dikes cut granite, the adjacent granite is enriched in muscovite and apatite, and depleted in K-feldspar. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that H, P, and locally B, Ti, Fe, and Ca were added, and K, Sr, Ba, and locally Na were removed (hydrogen metasomatism). In one alteration zone (8 cm wide) the calculated change (in terms of mols/gram of unaltered granite) is, 600 K-feldspar+24 biotite+190 plagioclase +[770 H+36 P+3 Ti+13 Fe+13 Ca] 400 muscovite+1100 quartz +11 apatite+[240 Na+260 K]. Where pegmatite dikes cut schist-hornfels (biotite-plagioclase-quartz), the adjacent rock is, in places, enriched in tourmaline, apatite, and quartz, and depleted in biotite and plagioclase. These alteration zones are variable in width; most are less than 20 cm wide. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that B, P, Zn, and locally Ca, Fe, and Al were added, and that Na, K, Fe, Rb, Sr, Ba, and locally Mg and Si were removed (boron metasomatism). In one zone, 2 cm wide, the calculated reaction (in units of mols/gram of unaltered schist) is, 730 biotite+1530 plagioclase +[1080 B+600 H+430 P+360 Ca] 480 tourmaline+480 quartz+115 apatite +[3630 Si+870 Na+590 K+110 Fe]. Changes in the volume fraction of muscovite, K-feldspar, tourmaline, and biotite, relative to distance from pegmatite, are progressive, and in most alteration zones may be expressed by use of an error-function equation. Some tourmaline zones are more complex. Zone formation is considered in terms of a steady-state reaction model in which grainboundary diffusion is the transport mechanism.     

The origin of banding in the Main Donegal Granite,N.W. Ireland

This Caledonian pluton exhibits a regular, sub-vertical banding in which layers of dark, relatively fine grained trondhjemite alternate with layers of light, coarse grained granite. The textures and the presence of cross-cutting bands of granite indicate that the light bands were derived by potash feldspathization of the trondhjemite. The geometry of the banding indicates that it formed during the “synplutonic” deformation of the pluton, largely in situ. The bands formed by segregation of alkalies into dilatant zones of movement produced during the deformation of partly consolidated magma.     

The origin of the albite-rich rocks enclosing the cobaltian pyrite deposit at Thackaringa,N.S.W., Australia

A stratabound disseminated and massive cobaltian pyrite deposit at Thackaringa, 30 km SW. of Broken Hill occurs in banded albite-quartz-biotite rocks which are conformable with regional structure and stratigraphy. The albite rocks are associated with pelitic and psammitic metasediments, amphibolite and minor quartzgahnite rocks. The deposit has undergone granulite facies metamorphism and there is no evidence that pyrite has reacted to form pyrrhotite. It is suggested that the albite rocks were an analcimized tuffaceous rock in which disseminated pyrite formed by a volcanic exhalative process. A massive cobaltian pyrite body is enclosed by a metamorphosed hydrothermal alteration zone characterised by an increase in quartz, magnesian fluorobiotite and Rb.